In modern motor vehicles, one significant aspect in the planning process is the design of the crumple zones, i.e. those regions or components of the vehicle which absorb energy through deformation in an accident and thus protect other vehicle regions and, especially, the occupants from damage or injury. In a typical design concept, the actual bumper is connected to the vehicle chassis by energy absorption elements (e.g. crash boxes, crash tubes or crumple cells). The bumper itself is of more robust design by virtue of its shape and material. The energy absorption elements, which extend approximately in the direction of travel, are less robust. These can be composed of aluminum or an aluminum alloy, for example, and deform relatively easily when a force is exerted on them by the bumper.
In the case of low-speed rear end collisions, the deformation can thus ideally be limited substantially to the energy absorption elements, while the vehicle chassis remains undamaged. This may considerably reduce repair costs. The bumper can optionally also be formed integrally with the energy absorption elements. Actual attachment to the vehicle chassis is generally accomplished by means of connecting plates that are welded to the energy absorption elements and can be connected to the vehicle chassis, for example to a longitudinal frame member, e.g. by a threaded fastener connection.
A heat-affected zone, in which stresses can arise in the material due to heating and cooling and there may also be unwanted changes at the microstructural level, forms on both sides of the weld seam during the welding of the energy absorption element to the connecting plates. The mechanical characteristics of the material in the heat-affected zone can change in a negative way relative to the parent material. As a result, cracks can form preferentially in the heat-affected zone and also propagate within the zone when a load is imposed. This can have the effect, for example, that the energy absorption element separates completely or at least partially from the connecting plate even though the actual weld seam remains intact. In this case, structural integrity, i.e. the mechanical connection formed by the bumper bar between the two longitudinal members, is no longer assured. Failure of the element can occur, for example, in the event of a collision with a fixed obstacle offset relative to the center of the vehicle (e.g. in an SORB test (small offset rigid barrier)), and this failure can be observed on the opposite side from the impact.
U.S. Pat. No. 1,885,398 A shows a method by which it is possible to repair cracks which form at an angle of a flange, for example. In a first step in this method, the region affected by the crack is burnt out electrically or thermally and, in a second step, it is filled by welding. In a third step, a metal piece having the cross section of a right angled triangle is welded into the angle.
U.S. Pat. No. 5,530,219 A describes a welded bracket which can be used to repair a reactor vessel in which cracks have formed due to stress corrosion. The bracket is screwed to the reactor vessel and comprises a plurality of annular parts, which are welded together in a special method. Two bracket segments to be joined are butted together, wherein the abutting ends have a kind of central projection, on both sides of which there remain gaps which are welded with a special blade type electrode during the welding process.
EP 1 674 187 A1 shows a welding method for producing a welded joint which prevents propagation of embrittlement cracks. For this purpose, a repair weld is made to a butt weld seam, the repair weld being harder than the butt weld seam. The repair weld is approximately triangular in shape, and a vertex of the triangle lies on the butt weld seam. The sides of the triangle enclose an angle of between 10° and 60° with the direction of extent of the seam. As an option, the repair weld can be combined with a reinforcing plate which rests by means of one edge on the butt welded workpiece and is welded to the latter.
CN 103192223 shows a method for treating cracks in a heat-affected zone of a weld seam. Here, an ultrasound analysis of the crack is first of all performed, after which material is either chiseled or milled out. In a subsequent step, holes are drilled at the ends of the crack. A repair weld is then carried out, this being preceded and followed in each case by heat treatment of the workpiece, during which thermal insulation is provided by means of rock wool.
In light of the indicated prior art, there is still room for improvement in the provision of a bumper unit with optimized crash behavior, especially with the aim of preventing unwanted severing of the joints to the vehicle structure.